An automatic transmission has internal clutches and bands that are immersed in the automatic transmission fluid (ATF). As the friction material on the clutches wear, it is carried by the ATF and tends to settle in tight passages and at the bottom of the transmission fluid pan as a sludge-like goo. A power flush could dislodge the goo from where it has settled and get it stuck in a tight orifice and cause shifting problems. If the transmission has been flushed routinely, then the goo doesn't have a chance to accumulate, but if there is a chance that there is a goo accumulation, a simple drain and refill is a safer course of action.
If you look at the owner's manual, the automaker doesn't even list a transmission flush in the recommended maintenance interval, just a drain and refill.